Pressurized water nuclear reactors include a core formed of assemblies which are prismatic in shape and which are disposed vertically and rest on a support plate within the chamber of the nuclear reactor.
During operation of the nuclear reactor, it is necessary to carry out periodically measurements of neutron flux within the actual core. For this purpose, use is made of fission detectors which have very small dimensions and which are displaced by remote control by means of flexible remote control cables, within tubes which are closed at one of their ends and which are called glove fingers. The glove fingers are introduced into certain fuel assemblies which are disposed in accordance with a predetermined distribution within the core. By displacing the flux detectors within the glove fingers introduced into the fuel assemblies, it is possible to carry out flux measurements over the entire height of the core. It must be possible for the glove fingers to be extracted from the assemblies of the core, for example in order to facilitate the operations of recharging the core of the reactor; in order to do this, traction is exerted on the end of the glove fingers, from an instrumentation which is disposed laterally relative to the chamber bore of the reactor.
The glove fingers must accordingly be guided from the instrumentation centre as far as the chamber, and then into the interior of the chamber, between the lower domed floor of this chamber and the entry end of the guide tubes of the corresponding assemblies. In order to do this, each one of the glove fingers is introduced into internal cylindrical and rectilinear passage of a guide assembly which includes, in particular, a guide tube passing into a connecting collar of the floor of the chamber, a guide column and an opening traversing the lower core plate in the extension of the guide tube of the assembly.
This cylindrical connecting passage of the glove finger has a diameter which generally decreases from the chamber floor to the fuel assembly, this diameter nevertheless remaining substantially greater than the external diameter of the glove finger, in order to permit facilitated introduction and displacement of this glove finger in the connecting passage.
The terminal part of the guide assembly is generally constituted by a sleeve fixed on the upper face of the core plate in the extension of the opening passing through this plate. A space is nevertheless provided betewen the end of this sleeve and the inlet of the guide tube of the fuel assembly, in order to facilitate the setting-up of this assembly on the occasion of the recharging of the reactor, and to permit the displacements accompanying the differential expansion of the fuel assembly relative to the lower internal equipment of the reactor.
The pressurized cooling water of the reactor, which water circulates at high speed in the chamber, causes the initiation of vibration of the glove fingers and impacts by these glove fingers against the walls of the internal passage of the guide assembly in which the glove finger is mounted with a certain play.
The pressurized water circulating in transverse directions acts on the glove finger, in the zone where the latter is not protected by a guide means, prior to entry thereof into the guide tube of the assembly. The pressurized water likewise passes through the internal cylindrical passage of the guide assembly in its axial direction, and this turbulent flow likewise generates vibrations and impacts by the glove finger against the walls of the connecting passage.
This results in excessive wear and possibly deteriorations of the glove fingers when the reactor is in service.